Re: [Amc-list] rear wheel lock up problem still

["Swygert, Francis G MSgt 436 CES/CECM" <Francis.Swygert3@xxxxxxxxxxxx>]

In my experience, John, none at all. I've used 78-81 Concord/Spirit 2.6"
calipers on four different disc brake upgrades. None had a balance valve
of any sort. The rear wheels would lock only under panic conditions in
the heavier cars. My 63 American and another American (62?) would lock
all four at the same time -- I think mainly due to the lighter weight of
the cars along with the short wheelbase. 

Dave's problem appears to be the PBR calipers and small piston area.
They need more pressure to work correctly. Once that is fixed, the rears
may not lock so early. Jeff Puras sent me a note stating that when the
fronts aren't working to full potential the rears, trying to do more
with less weight on them, will lock up prematurely. I didn't think about
that! My long-winded posts on brakes were to illustrate that everything
has to be taken into account when modifying such a critical system --
see how easy it is to not think about something? 

The Americans I upgraded would only lock on a slippery surface under
panic conditions, it wasn't easy to lock them! Locking was a
contributing factor in one accident with my car, but I don't think it
would have helped had they not locked. It had just rained, and I was on
a bridge off-ramp going down hill when a car unexpectedly locked its
brakes in front of me. There were four people in the car (including me)
and I stood on the brakes with both feet. I let off at the last instant
and turned into a parking lot, just about ripping my left front fender
off on the 80s full size LTD's bumper. The LTD was still on the brakes
-- the bumper caught my American at the lower edge of the headlight. You
know how high one of those headlights are? I was off the brakes -- the
front end of my car had come back up!! As soon as I saw I wasn't going
to be able to stop I looked for a way out -- motorcycle riding forces
you to do that! A real bike rider is one of the best drivers... I
haven't been riding the bike much lately though. Afghanistan for the
summer, back just in time for bad weather!! 

I drove my Classic for a while with no balance valve with the stock
drums and 2.6" calipers, stock master cylinder (with residual pressure
valve removed). I had no problems, but on slick surfaces the rears would
lock noticeably before the fronts. It wasn't easy, but hard braking (not
quite panic, but close -- as hard as I could with one foot on the brake
would do it, didn't need both feet) on slick surfaces would lock the
rears. On a sedan it would likely be a bit worse than the wagon, but
then the sedan has a smaller wheel cylinder -- may act the same. I
didn't install a proportioning device because I intended to put on rear
disc brakes from the start and I knew I could handle the car if the rear
did lock. Well, I know how to keep it from locking and did some testing
-- I knew how hard I could brake without locking. TESTING IS IMPORTANT
WHEN CHANGES HAVE BEEN MADE!! Dirt road or wet parking lot -- both seem
to work about the same. The point is you need to KNOW what the car will
do in wet weather. 

----------------------------------------
Date: Thursday, February 15, 2007 12:11 AM
From: John Elle <johnelle@xxxxxxxxx>

Now for the $64.00 question of the moment.
What kind of problem would one have expected to have to deal with if
1980 or so Concord/Spirit front discs had been used which are a complete
bolt on as far as mounting go and considering that the rear wheel
cylinder is 15/16 on them?
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